This invention relates to improved insulative coatings for electrical steels, more particularly to an aqueous coating composition which does not contain colloidal silica or chromic acid for forming a tension producing insulative coating which provides improved core loss in the electrical steels.
As used herein the terms "electrical steel" and "silicon steel" relate to an alloy which may have a typical but non-limiting composition, in weight percent, of about 0.06% maximum carbon, about 4% maximum silicon, about 0.03% maximum sulfur or selenium, about 0.02% to 0.4% manganese, about 0.4% maximum aluminum, and balance essentially iron.
The insulative coatings of the present application can be applied to carbon steels for electrical uses, non-oriented silicon steels and silicon steels having various orientations. The coating solutions of the present invention may be applied to silicon steels with or without a mill glass base coating. The invention has particular utility for application to cube-on-edge oriented silicon steel of regular grade or high permeability grade, wherein the body-centered cubes making up the grains are oriented in a position designated at (110)[001] in accordance with Miller's indices. As is well known in the art cube-on-edge oriented silicon steel sheet has numerous uses, such as in laminated magnetic cores for power transformers and the like.
In the manufacture of cube-on-edge oriented silicon steel an annealing separator is used during the final anneal to which the silicon steel strip or sheet is subjected, and if a magnesia or magnesia-containing annealing separator is used, a glass film is formed upon the surfaces of the strip or sheet, which is generally known in the industry as "mill glass".
So-called secondary coatings are also well known in the art which are used in addition to or in place of a mill glass coating. Such secondary coatings generally impart tension because of thermal expansion differences. Such tension refines the domain wall spacing which in turn results in improved magnetic quality in the silicon steel strip and sheet.
U.S. Pat. Nos. 3,996,073 and 3,948,786 disclose insulative coatings which may be used in addition to or in place of a mill glass on silicon steel strip. A coating solution, in accordance with these patents, contains aluminum, magnesium and phosphate in the following relative relationship on a water-free basis:
From 3 to 11% by weight Al.sup.+++ calculated as Al.sub.2 O.sub.3, from 3 to 15% by weight Mg.sup.++ calculated as MgO and from 78 to 87% by weight H.sub.2 PO.sub.4.sup.- calculated as H.sub.3 PO.sub.4, with the concentration of Al.sup.+++, Mg.sup.++ and H.sub.2 PO.sub.4.sup.- comprising 100 parts by weight calculated as aluminum oxide, magnesium oxide and phosphoric acid respectively on a water-free basis.
The solution further contains from 0 to 150 parts by weight of colloidal silica on a water-free basis, at least 45% by weight of the coating solution being water. When colloidal silica is present, within the range of 33 to 150 parts by weight on a water-free basis, at least 60% by weight of the coating solution must be water, and from 10 to 25 parts by weight chromic anhydride for every 100 parts by weight H.sub.2 PO.sub.4.sup.-, calculated as H.sub.3 PO.sub.4, are needed in order to stabilize the colloidal silica and to provide satisfactory adherence, lack of hygroscopicity and "tack" after curing. When applied as a secondary coating in addition to a mill glass base coating, the insulative coating of these patents imparts tension to electrical steel strip, thereby improving magnetic properties.
U.S. Pat. Nos. 3,501,846 and 2,492,095 disclose phosphate coatings for silicon steels.
Magnesium phosphate based and aluminum phosphate based secondary coatings are disclosed in U.S. Pat. Nos. 2,743,203; 3,151,000; 3,594,240, 3,687,742, and 3,856,568.
U.S. Pat. No. 3,649,372 discloses a composition for an applied insulative coating comprising mono-basic magnesium phosphate, aluminum nitrate and/or aluminum hydroxide, together with chromic anhydride.
Belgian Pat. No. 789,262 discloses an applied insulative coating obtained from a solution of mono-aluminum phosphate, colloidal silica and chromic acid or magnesium chromate. This is alleged to be a tension-imparting film.
U.S. Pat. No. 3,948,786 states that the presence of colloidal silica in the composition may be required in order to prevent adherence of the applied coatings to furnace rolls in a conventional roller hearth furnace used for thermal flattening. However, whenever colloidal silica is added without chromic anhydride the stability of the aqueous solution is adversely affected, i.e. the composition increases in viscosity with time and may form a gel. The addition of chromic anhydride was found to stabilize the solution, but chromic anhydride is very expensive and is toxic.